Very subjective question. I had the galaxy s (1) and I didn't like it due to poor battery life and it crashed sometimes, and some apps didn't work on it.... I had the galaxy s (2) and it was much better. The battery life on it was still a concern and apps sometimes would crash.... I now have the nexus and apps do not crash!! It is heavier and larger, but it also follows the new layout standard that Google is pushing, which I like very much. Everything feels much more intuitive.
What tilted me to the nexus, 1) you don't need to install a custom from to get stability and good battery life, and 2) it gets updates the quickest.
People will tell you the screen isn't as great and the S2 has a better camera. This is true, but the camera is better than that of the s1 and the screen is still very good. In sum, for hardware, the S2 wins, but for software the nexus wins.
In all honesty, the s2 probably won't get an update past android 4.0... Samsung has shown that if an OS isn't released by Google with 12 months of the release of their phones, then they wont implement the update. The nexus line gets 18months....
Sent from my Galaxy Nexus using Tapatalk 2